The present invention relates generally to a turbo-electric machine for power generation, having rotor and stator windings, and particularly relates to apparatus for maintaining wedges and amortisseurs in key slots overlying the windings slots and other parts in efficient, sufficient, low-resistance, electrical contact with one another and the rotor body during static start or very low rotor speeds. The present invention also relates to a tool for and method of inserting the wedges in the key slots, and a method of operating the machine at start-up and low rotor speeds.
In conventional power generators, the rotor is provided with circumferentially spaced, axially extending slots which receive the rotor windings. The windings are retained in the rotor slots by wedges disposed in key slots radially outwardly of the windings. In conventional turbo-electric machines, a starter motor drives a turbine, typically a gas turbine, up to 65-70% of its operating speed, at which time the turbine becomes operational to drive the generator. When the generator supplies power to the power system, the current which flows through the stator windings sets up rotating magnetic fields which induce a substantial amount of current on the surface of the rotor. These surface currents will flow along various paths, including the rotor wedges, rotor body, amortisseur, and end retaining rings. The centrifugal forces generated at normal operating speeds maintain efficient, low-resistance electrical contact between the various rotor surface current carrying parts, whereby arcing between these parts and localized heating due to the induced currents are avoided.
However, in a static start of the generator where it is employed as the starter motor for the turbo-electric machine, typically to bring the gas turbine up to speed, current is supplied to the motor (generator) windings at a time when the rotor is stationary or rotating at a very low speed, for example, on the order of 3 or 4 r.p.m. This similarly induces substantial surface current in the rotor and associated parts. Under these conditions, the centrifugal forces are virtually non-existent or, at least, ineffective to force sufficient electrical contact between rotor parts, for example, between the wedges and the rotor slot surfaces, the amortisseur and the wedges and the amortisseur and the end retaining rings. When substantial induced currents at low r.p.m. flow between rotor parts having gaps or poor contact at their interfaces and hence areas of high resistance, arcing and/or localized heating may occur, causing severe damage to the rotor parts.
To prevent such damage at low r.p.m., insulation may be provided to preclude the induced current flow in discrete zones of interfacing rotor parts. However, this is very difficult to accomplish and is mostly impractical. However, it has been found that radial outward biasing forces, similar to those obtained by centrifugal action when operating at normal speeds, may be provided on the various overlapping faces between the amortisseur, wedges, rotor body, and retaining rings to afford sufficient induced electric current carrying capability at the interfaces of these parts at generator start-up and low rotor speeds. The present invention is directed to apparatus and methods for accomplishing this, apparatus for effectively inserting the wedges in the key slots taking into account the apparatus providing the biasing forces at start-up and low rotor speeds, and methods of assembly.
According to the present invention, there is provided a generator rotor construction in which the rotor parts in the key slots over the rotor slots above the rotor windings are maintained under radial outward pressure against themselves, the rotor body, and the retaining rings, and, hence, lie in efficient electrical contact at static start or very low rotor speeds. This construction also enables the centrifugal forces at normal operational speeds to provide efficient, low-resistance, electrical contact between such parts and the rotor body and retaining rings. To accomplish this, and in accordance with the present invention, elongated strip-type springs are disposed in the key slots underlying the amortisseur and the wedges. The spring strips maintain sufficient radial outward pressure on the amortisseur and wedges to allow the induced surface current during start-up and low rotor speed to flow across the resultant low-resistance interfaces of such parts thereby preventing arcing, localized overheating and damage to the rotor parts.
Each spring strip comprises an elongated piece of conductive or non-conductive material which is arcuate in transverse cross-section. In the slot keys, the spring strips have elongated edges which bear on shoulders at the base of the key slots. Because of the curvature of the spring strips, they bear upwardly against the underside of an elongated amortisseur which, in turn, underlies the wedges and the retaining rings. The slot amortisseur is an integral one-piece construction and extends the entire length of the windings slots. Additional amortisseur pieces are used at the ends of the rotor body underneath the retaining rings and comprise short fingers which overlap the integral slot amortisseur that runs the entire length of the rotor. The wedges and springs are provided in discrete lengths in the key slots. The spring strips also have one or more apertures therethrough for conducting rotor cooling gas, the apertures lying in registry with corresponding apertures through the amortisseur and wedges. Thus, the spring strips bias the rotor parts in the key slots radially outwardly and, hence, into low-resistance contact, at the interfaces between these rotor parts, along the length of the key slots, particularly at generator (motor) start-up and low speeds. These interfaces lie between the rotor body and wedges, the amortisseur and wedges, the spring strips and the amortisseur, the rotor body and the spring strips, and, at the rotor ends, between the amortisseur and amortisseur finger and the end retaining rings. It will be appreciated, however, that the centrifugal forces extant at operational speeds will force the rotor parts in the key slots into contact with one another and the rotor body, and, at the rotor ends, into contact with the retaining rings, thereby affording efficient low-resistance induced surface current flow paths between rotor parts.
Surface currents during start-up and low rotor speeds will also flow through throttled areas between the ends of cross-slots in the rotor and the windings slots. With such high currents, these confined areas may become overheated with ensuing damage. In order to accommodate these high induced currents at start-up and low rotor speeds, the present invention provides cross-slots which are reduced in depth and are axially spaced closer to one another than in a conventional rotor construction to provide the required flexibility needed for rotor body bending symmetry as the rotor rotates. Thus, for example, axial spacing between the cross-slots of 12 inches in prior rotors has been reduced to approximately 8 inches. The more shallow depth of the cross-slots enables the distance between the ends of the cross-slots and the winding slots to increase, for example, on the order of about 2 to 3 inches.
There is also provided in accordance with the present invention a tool for assembling the wedges in the key slots. It will be appreciated that the spring strips and amortisseur are necessarily disposed in the key slots before the wedges are inserted into the key slots. This requires the wedges to be inserted in the confined space between the wedge-retaining surfaces of the key slot of the rotor body and the amortisseur and against the bias of the spring strips. The tool comprises a pair of tool dogs, one of which is pivotally carried on a main arm. One dog is designed to have a cross-section at its distal end corresponding to the upper portion of the key slot and is rotatable for insertion into the key slot. The second dog has a generally T-shaped cross-section for abutment against the outer margins of the rotor body which straddle the key slot. Thus, by forcing the main arm downwardly, the dogs fix the tool in a predetermined axial position along the rotor. A second arm is pivotally attached to the main arm and includes a driving head for engaging an end of a wedge. The underside of the wedge adjacent one end is provided with a shallow, tapered surface to facilitate initial insertion of the wedge into the key slot. Thus, when the wedge has been started in the slot and the tool dogs fix the tool to the rotor body at a selected axial position along the key slot, the second arm is rotated to engage the head against the wedge and, upon further rotation, advances the wedge along the key slot. This operation is repeated until the wedge is disposed in its predetermined axial position.
In a preferred embodiment according to the present invention, there is provided an electric machine having a rotor body, elongated axially extending slots in the rotor body for containing rotor windings and elongated axially extending slot keys overlying the windings slots. A rotor parts assembly is provided comprising at least one wedge disposed in each key slot overlying the windings slot, and an elongated spring strip having a length greater than its width is disposed between the wedge and the body in each key slot and extends generally in the axial direction for biasing the wedge in a radial outward direction.
In a further preferred embodiment according to the present invention, there is provided a rotor for an electric machine, comprising a rotor body having elongated axially extending slots for containing rotor windings and elongated axially extending slot keys overlying the windings slots, each key slot having a pair of contact surfaces facing generally inwardly of the rotor, a plurality of wedges disposed in each key slot overlying the windings slot with each wedge having a pair of surfaces in opposition to the pair of surfaces of the key slot, and a plurality of elongated spring strips, each having a length greater than its width, disposed in each key slot and extending generally in the axial direction for biasing the wedge in a radial outward direction to engage the pairs of surfaces and provide low electrical resistance contact paths between the engaged pairs of surfaces.
In a still further preferred embodiment according to the present invention, there is provided a tool for applying wedges in key slots overlying windings retaining slots in a rotor body of an electric machine wherein the key slots have at least one spring element for biasing the wedge in the key slot into high-pressure, efficient electrical contact with the rotor body, comprising a lever having a main arm and a pair of dogs carried by the lever adjacent an end of the main arm, one of the dogs shaped for disposition in the key slots and the other dog shaped to overlie the key slots whereby pressure applied to the main arm, with the one dog in a key slot and the other dog overlying the key slot, applies pressure to the dogs in opposite directions, thereby fixing the lever at a predetermined axial position along the key slot, and a second arm pivotally carried by the lever and carrying a head for engaging the wedge whereby, upon fixing the lever at a predetermined axial position along the key slot, the second arm may be pivoted to advance a wedge along the key slot.
In a still further preferred embodiment according to the present invention, there is provided a method of assembling rotor parts in the rotor body of a turbo-electric machine for affording efficient electrical contact between the rotor surface current carrying parts at machine start-up or low rotor speeds wherein the rotor body has key slots for receiving spring strips, an amortisseur and wedges for retaining rotor windings in underlying windings slots, comprising the steps of (a) inserting an amortisseur into a key slot, (b) inserting a plurality of spring strips between the amortisseur and the base of the key slot, (c) inserting a wedge into the key slot to overlie the amortisseur, (d) advancing the wedge longitudinally along the key slot until the wedge reaches a location therealong adjacent an underlying spring strip, (e) incrementally advancing the wedge along the key slot to overlie the spring strips and (f) repeating steps (b)-(e) until the wedges, amortisseur and spring strips each extend the full longitudinal length of the key slot.
In a still further preferred embodiment according to the present invention, there is provided a method of starting a generator or operating the generator at low rotor speeds wherein efficient electrical contact is afforded between rotor surface current carrying parts at start-up and low rotor speed, the generator having a rotor body, elongated axially extending slots in the rotor body for containing rotor windings and elongated axially extending key slots overlying the windings slots, comprising the step of biasing (1) an elongated amortisseur disposed in each key slot and having a length coextensive therewith in a radial outward direction against the underside of a plurality of wedges in each key slot overlying the corresponding amortisseur and (2) the plurality of wedges radially outwardly against the rotor body, into continuous electrical contact along the amortisseur, wedges and rotor body at their respective interfaces wherein arcing and localized heating is substantially precluded at the respective interfaces during generator start-up and low rotor speeds due to induced surface current flow across the interfaces.
Accordingly, it is a primary object of the present invention to provide novel and improved apparatus and methods for maintaining high pressure, low resistance and hence, efficient electrical contact, between rotor parts during static start or low rotor speeds in a turbo-electric machine. It is also an object of the present invention to provide a novel and improved tool for assembling the wedges in the key slots of the windings slots overlying the windings and a method of assembly.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.